(1). Field of the Invention
This invention relates to an X-ray CT apparatus including an X-ray source, and an X-ray detector opposed thereto and having an array of numerous X-ray detecting elements, the X-ray source and X-ray detector being revolvable together about an object under examination or a patient to obtain sectional images of the patient.
(2). Description of the Related Art
In an X-ray CT apparatus, a temperature increase in an X-ray tube results in variations in the incidence position of X rays. Then, the sensitivity of X-ray detecting elements varies to give rise to the problem of artifacts appearing in the sectional images obtained. Various causes are conceivable for such variations in the sensitivity of the X-ray detecting elements. In the case of a solid-state detector, such variations may be due to the transmittance or cementing of a scintillator or to an inconstant parallelism of a collimator. In the case of a gas detector of the ionization chamber type, such variations may be caused by each channel having a varying width in the direction of a body axis.
In one conventional technique, therefore, the sensitivity of each detecting element relative to a temperature inside the tube is measured in advance based on scanning conditions and downtime of the tube. A sensitivity level is appropriately selected according to a use situation for actually scanning a patient, in order to correct data obtained from the patient. In another conventional technique, a monitoring detector such as a two-dimensional detector or a line sensor having a resolution in the direction of the patient's body axis is additionally provided to detect a focal shift of the X-ray tube. When the patient is actually scanned, data obtained from the patient are corrected with a predetermined sensitivity correction value according to an X-ray incidence position detected by the monitoring detector.
However, where a correction is made based on a temperature in the tube, actual amounts of focal shift are not detected. Especially, in the event of hysteresis occurring between temperature increase and focal shift of the X-ray tube, it is impossible to make a satisfactory correction of the sensitivity of each X-ray detecting element.
Where a monitoring detector is additionally provided, the apparatus has a complicated overall construction with additional control circuits and so on.